Intermittent Fasting Facts

Intermittent Fasting Explained: 14:10, 16:8, OMAD and the Truth About Fasting

Are 16-hour fasts really better than 14 hours? Is eating once a day healthier? What about 2 meals versus 3? We look at what the science actually says about intermittent fasting, time-restricted eating and meal frequency.

Intermittent fasting has become one of the biggest trends in nutrition.

Search online and you will find claims that fasting can “switch on fat burning”, activate autophagy, reduce visceral fat, reverse ageing and dramatically improve metabolic health.

At the other extreme, you will find people saying that fasting is simply another way of restricting calories and that there is nothing special about it.

So which is right?

The answer, as is often the case with nutrition, lies somewhere in the middle.

There is good evidence that intermittent fasting and time-restricted eating can help with weight loss and may improve some measures of metabolic health.

However, there is far less evidence that a particular fasting period – such as 16, 18 or 20 hours – is uniquely beneficial.

Nor is there convincing evidence that one meal a day (OMAD) is superior to two or three nutritious meals.

The biggest lesson from the research is surprisingly simple:

Fasting can be a very useful tool, but it is not a metabolic magic trick.

What Is Intermittent Fasting?

Intermittent fasting is an eating pattern that deliberately alternates between periods of eating and periods of fasting.

There are several different approaches.

Common fasting schedules

The two approaches that receive the most attention are time-restricted eating, where food is consumed within a daily window, and intermittent fasting approaches such as 5:2 or alternate-day fasting.

The National Institute on Aging distinguishes these approaches and points out that, despite considerable research, important questions about their long-term benefits and safety remain unanswered, particularly in older adults.

Does Intermittent Fasting Actually Work?

Yes.

But there is an important qualification.

Intermittent fasting can help people lose weight, but current evidence does not suggest that it is dramatically superior to every other effective calorie-controlled diet.

A major 2025 systematic review and network meta-analysis published in The BMJ examined 99 randomised clinical trials involving 6,582 adults.

The researchers found that intermittent fasting strategies and conventional continuous calorie restriction both reduced body weight compared with unrestricted eating.

When intermittent fasting was compared directly with continuous calorie restriction, alternate-day fasting was the only intermittent-fasting strategy that produced a statistically significant additional reduction in body weight, and the difference was modest, about 1.3 kg on average.

That is a very important finding.

It tells us that fasting works.

But it also tells us that we shouldn’t confuse:

“This works”

with:

“This is vastly better than everything else.”

Why Does Fasting Help People Lose Weight?

One of the simplest explanations is that a shorter eating window can make it easier to consume fewer calories.

Imagine someone who normally eats:

• Breakfast
• Mid-morning snack
• Lunch
• Afternoon snack
• Dinner
• Evening snacks

Move that person to a 10-hour eating window and several of those eating opportunities may disappear.

The result can be a reduction in total calorie intake without deliberately counting every calorie.

This is one reason intermittent fasting can be so effective for some people.

There is another important point, however:

Fat loss still requires an overall energy deficit.

Fasting does not repeal the laws of energy balance.

You can fast for 16 hours and still consume enough calories during your eating window to maintain or gain weight.

Equally, someone eating three meals per day can lose substantial amounts of body fat if their overall diet produces a sustainable calorie deficit.

Is a 16-Hour Fast Better Than a 14-Hour Fast?

This is one of the most common questions about fasting.

The honest answer is: not necessarily.

The popularity of 16:8 has led many people to believe that 16 hours is some kind of biological threshold.

It isn’t.

There is no evidence that your body suddenly changes from one metabolic state to another precisely when the clock reaches 16 hours.

A 2020 randomised clinical trial involving 116 adults with overweight or obesity compared an 8-hour time-restricted eating pattern with consistent meal timing.

The time-restricted eating group lost a modest amount of weight, but the difference from the control group was not statistically significant.

This does not mean 16:8 is ineffective.

It means something more useful:

There is no good reason to believe that everybody needs exactly 16 hours of fasting.

For some people, 14 hours may be easier to maintain and may produce excellent results.

What About 18:6 or 20:4?

As the fasting window becomes longer, the eating window becomes increasingly restrictive.

That may help some people naturally reduce calorie intake.

But there is currently no strong evidence showing that moving from 16:8 to 18:6 or 20:4 produces a proportionally greater health benefit.

In fact, increasingly restrictive eating windows create another problem:

Can you still eat a nutritionally complete diet?

As the eating window becomes smaller, you have less time to consume:

• Sufficient protein
• Vegetables
• Fruit
• Fibre
• Healthy fats
• Essential vitamins and minerals
• Adequate overall calories

This may not matter much to someone eating a large amount of food during a four-hour window.

But for someone deliberately restricting calories, it can become a genuine nutritional challenge.

Is Eating Once a Day Healthier?

This is where fasting enthusiasts sometimes get carried away.

There is currently no convincing evidence that eating one meal a day is inherently healthier than eating two or three well-constructed meals.

OMAD can certainly produce weight loss in some people.

Why?

Because consuming an entire day’s food in one sitting can make it difficult to eat as much as you would have spread over a longer period.

But that doesn’t mean the one meal has a special metabolic property.

A systematic review and network meta-analysis examining meal frequency found little robust evidence that simply reducing the number of meals provides a major advantage.

Another systematic review found no discernible advantage of higher versus lower meal frequency for weight change or cardiometabolic health, although the certainty of the evidence was very low.

So the evidence does not justify the claim:

“One meal a day is the optimal way for humans to eat.”

Two Meals a Day vs Three Meals a Day

This is much less dramatic than the internet makes it sound.

There is no universal biological requirement to eat exactly three meals every day.

Likewise, there is no compelling evidence that everyone should reduce their eating to two meals.

Two meals a day may work well if:

• You aren’t particularly hungry between meals.
• You can comfortably consume enough protein.
• Your meals contain plenty of vegetables and fibre.
• You don’t become excessively hungry later.
• It makes controlling your calorie intake easier.
• It fits your lifestyle.

Three meals a day may work better if:

• You prefer regular meals.
• Large meals are uncomfortable.
• You exercise regularly.
• You need to distribute protein across the day.
• Two large meals leave you constantly hungry.
• Three meals make the diet easier to sustain.

The important point is that meal frequency should serve the diet, not become the diet.

Does Fasting Burn More Fat?

Yes, but this is one of the areas where online fasting claims often exaggerate the science.

During fasting, insulin levels generally fall and the body’s use of stored fat for energy increases.

That is a genuine physiological response.

But there is a crucial distinction:

Increased fat burning during a fasting period does not automatically mean greater long-term fat loss.

Your body continually adjusts its fuel use according to food intake and energy expenditure.

Someone can burn more fat while fasting and then replace that energy later by eating more.

Therefore, the important measurement is not:

“How much fat am I burning at 3pm?”

It is:

“What is happening to my body composition over weeks and months?”

That is the number that matters.

What About Visceral Fat?

Visceral fat is the fat stored around internal organs and is associated with increased cardiometabolic risk.

This is particularly important because losing abdominal fat is one of the major reasons people become interested in fasting.

Research has found that time-restricted eating can reduce body weight, waist circumference and fat mass.

A 2022 randomised clinical trial of early time-restricted eating found greater weight loss than a comparison eating schedule when both groups also received calorie-restriction treatment. The study did not, however, demonstrate a dramatically greater reduction in body fat.

More recent meta-analysis also suggests that time-restricted eating, lower meal frequency and earlier distribution of calories may produce modest reductions in body weight, but the researchers cautioned that the effects were small and the evidence had important limitations.

The practical message?

Fasting may help reduce visceral fat because it can help reduce overall body fat.

But there is no scientifically established fasting duration that specifically “targets visceral fat”.

The popular claim that:

“Visceral fat starts burning after exactly 16 hours”

is an oversimplification.

Does the Time of Day Matter?

Possibly.

This is one of the more interesting areas of current research.

Your body has circadian rhythms that influence metabolism, hormones and other physiological processes.

Some research suggests that eating earlier in the day may be preferable to consuming a large proportion of your calories late at night.

A randomised clinical trial involving 90 adults with obesity compared early time-restricted eating – an 8-hour eating window from 7am to 3pm – with eating over at least 12 hours.

The early eating group lost more weight over 14 weeks, although the difference in fat loss was less clear.

This suggests that the question may not simply be:

“How long should I fast?”

It may also be:

“When should I eat?”

There is increasing interest in the idea that eating earlier and avoiding very late-night meals may have advantages.

However, the evidence is still developing.

What Is Autophagy and Does Fasting Activate It?

Autophagy is a genuine cellular process.

In simple terms, cells break down and recycle damaged or unnecessary components.

Fasting and changes in nutrient availability can influence biological pathways associated with autophagy.

The problem is that internet discussions often turn this complicated biology into a stopwatch.

You will sometimes see claims such as:

12 hours = fat burning

16 hours = autophagy begins

24 hours = major cellular cleansing

Human biology doesn’t work that neatly.

There is currently no reliable human evidence establishing a universal fasting time at which autophagy suddenly “switches on” in some dramatic fashion.

Therefore:

Autophagy is real.

The exact fasting timetable promoted online is not established.

Does Fasting Increase Longevity?

This is perhaps the biggest claim made about fasting.

And it is important to be honest.

We do not currently know whether intermittent fasting makes humans live longer.

Much of the excitement surrounding longevity and fasting comes from animal and laboratory research.

These studies are interesting and may help explain biological mechanisms involved in ageing.

But findings in laboratory animals cannot automatically be translated into human lifespan.

The National Institute on Aging states that researchers still have much to learn about the long-term effects of fasting and calorie restriction in humans, particularly in older adults.

So be suspicious when you see:

“Science proves fasting extends human lifespan.”

It doesn’t.

A more accurate statement would be:

Fasting may influence biological processes associated with ageing, but whether it extends human lifespan remains unproven.

What Happens to Muscle During Fasting?

This question deserves much more attention.

When somebody loses weight, the weight lost can include:

Fat + water + lean tissue.

The objective shouldn’t simply be to make the number on the bathroom scales fall.

A much better objective is:

Lose excess fat while preserving as much muscle as possible.

This becomes increasingly important with age.

Older adults need adequate protein, and resistance training becomes particularly valuable when losing weight.

ESPEN guidance recommends at least 1.0 gram of protein per kilogram of body weight per day for older adults, with 1.0–1.2 g/kg/day commonly suggested for healthy older people. Requirements can be higher in some situations.

This is one reason I would be cautious about extremely restrictive fasting schedules.

The fewer opportunities you have to eat, the harder it may become to obtain enough protein and other nutrients.

Is OMAD a Good Idea for Older Adults?

There is an important distinction here.

Something can be possible without necessarily being optimal.

Someone may successfully consume all their daily calories and protein in one meal.

But for many older adults, a two- or three-meal pattern may make it easier to:

• Consume adequate protein
• Eat sufficient vegetables and fibre
• Distribute protein across the day
• Support resistance exercise
• Avoid excessive hunger
• Maintain a nutritionally complete diet

The evidence base for fasting specifically in older adults is still limited.

The National Institute on Aging notes that many fasting studies have been conducted in younger adults and that more research is needed to determine long-term effectiveness and safety in older people.

That doesn’t mean older people cannot fast.

It means extreme fasting deserves more thought than simply copying a fasting routine from a 30-year-old fitness influencer.

Are There Side Effects From Intermittent Fasting?

Intermittent fasting is generally well tolerated in clinical trials involving adults with overweight or obesity.

A 2024 systematic review and meta-analysis of 15 randomised trials involving 1,365 adults found no significant difference between intermittent fasting and control groups in fatigue, headache or study dropout.

There was a possible increase in dizziness in some analyses, although it did not reach conventional statistical significance.

Potential problems can nevertheless include:

• Hunger
• Headaches
• Dizziness
• Constipation
• Irritability
• Low energy
• Difficulty exercising
• Overeating during the eating window
• Difficulty obtaining adequate nutrition

Individual responses vary.

Anyone taking medication affected by food intake, blood glucose or blood pressure should discuss substantial dietary changes with a healthcare professional.

What Is the Best Fasting Window?

This is where I think the answer becomes much more practical.

12:12

Very easy to follow.

A sensible starting point for someone who dislikes the idea of fasting.

14:10

An excellent compromise.

A 14-hour overnight fast with a 10-hour eating window provides meaningful structure without making the diet excessively restrictive.

16:8

Probably the best-known time-restricted eating pattern.

It can work very well, especially for people who naturally prefer skipping breakfast or finishing dinner earlier.

But there is no evidence that the number 16 is magical.

18:6

More restrictive.

Potentially useful for some people but increasingly difficult to fit a balanced diet into.

20:4

Very restrictive.

There is little reason for most people to assume this is necessary for health or fat loss.

OMAD

The most restrictive of the popular approaches.

It may work for certain individuals, but it is not established as superior and can make nutritional adequacy more difficult.

So What Is the Best Approach?

Instead of asking:

“What is the longest fast I can manage?”

a better question is:

“What eating pattern can I maintain for years while losing excess fat, protecting muscle and enjoying my life?”

That completely changes the decision.

A fasting schedule should ideally:

1. Help control calorie intake.
2. Provide adequate protein and nutrients.
3. Support exercise.
4. Control rather than increase hunger.
5. Fit comfortably into your social life.
6. Be sustainable for years rather than weeks.

That is a much better definition of an optimal diet.

The Fasting Myth-Busting Cheat Sheet

Myth: “You must fast for 16 hours to burn fat.”

Reality: Your body uses a mixture of fuels continuously. Fasting increases fat oxidation, but 16 hours is not a magical threshold.

Myth: “Nothing happens until hour 16.”

Reality: Metabolic changes during fasting are progressive, not an on/off switch.

Myth: “OMAD is the ultimate fat-loss strategy.”

Reality: OMAD can reduce calorie intake, but it has not been established as superior to other sustainable approaches.

Myth: “The longer the fast, the better.”

Reality: Longer fasting isn’t automatically better and may make adequate nutrition more difficult.

Myth: “Fasting increases human lifespan.”

Reality: This remains unproven.

Myth: “You can’t lose weight without fasting.”

Reality: Completely false. Conventional calorie-controlled diets can work extremely well.

Myth: “Fasting specifically targets belly fat.”

Reality: Fasting can help reduce overall body fat and waist circumference, but there is no established fasting duration that selectively targets visceral fat.

What Matters More Than the Fasting Window?

When looking at the big picture, I would put the priorities in this order:

1. Diet quality

Eat predominantly nutritious, minimally processed foods and obtain plenty of vegetables, fibre, healthy fats and quality protein.

2. Total calorie intake

If fat loss is the objective, a sustainable calorie deficit is fundamental.

3. Protein

Especially important when losing weight and particularly important as we get older.

4. Resistance exercise

One of the most useful tools for preserving muscle during weight loss.

5. Daily movement

Walking and other physical activity support cardiovascular health, fitness and energy expenditure.

6. Sleep and recovery

These are often overlooked in weight-management discussions.

7. Meal timing

Useful and potentially beneficial, but probably less important than the fundamentals above.

8. Extreme fasting

Usually unnecessary.

The Bottom Line: Is Fasting Worth Trying?

Yes, for the right person.

Intermittent fasting can be an effective way of structuring your diet.

It can make calorie control easier.

It may produce modest improvements in weight, waist circumference, blood pressure and certain metabolic measures.

Eating earlier may also have advantages over eating large amounts of food late at night.

But the evidence does not show that everyone needs a 16-hour fast, that OMAD is superior, or that longer fasting is automatically healthier.

The most comprehensive recent evidence points towards a relatively sensible conclusion: intermittent fasting works, but its advantages over conventional calorie restriction are generally modest.

And that may actually be good news.

You don’t have to make eating complicated.

You don’t have to fast for 20 hours.

You don’t have to eat once a day.

You don’t have to fear carbohydrates after 6pm.

And you certainly don’t have to rearrange your entire life around a stopwatch.

A sensible starting point for many people may be a simple 14:10 or 16:8 pattern, combined with a nutritious diet, appropriate calorie intake, adequate protein and regular exercise.

The best diet isn’t the one with the most impressive fasting window.

It’s the one you can still be following five years from now.

Frequently Asked Questions

Is 14:10 fasting effective?

Yes. A 14-hour fasting period and 10-hour eating window is a reasonable form of time-restricted eating. It may help reduce calorie intake and can be easier to sustain than more restrictive approaches. There is no strong evidence that everybody needs to fast for 16 hours.

Is 16:8 better than 14:10?

Not necessarily. Although 16:8 is widely studied and popular, there is no established biological threshold at 16 hours that makes it automatically superior to 14:10.

Is OMAD better than 16:8?

There is no convincing evidence that one meal a day produces superior long-term results. OMAD may help some people control calories, but it can make adequate protein and nutrient intake more difficult.

Can fasting reduce visceral fat?

Fasting can contribute to reductions in body weight, waist circumference and body fat. However, there is no proven fasting period that specifically targets visceral fat.

Does fasting activate autophagy?

Fasting influences cellular pathways involved in autophagy, but there is not enough human evidence to establish an exact fasting duration at which autophagy suddenly “switches on”.

Can you drink coffee while fasting?

It depends on how strictly you define fasting. Plain black coffee contains very few calories and is commonly included during a fasting period. Adding sugar, milk or cream technically introduces calories and therefore changes the fast.

Can you drink water while fasting?

Yes. Water is important during fasting. Unless a medical procedure requires otherwise, hydration should not normally be deliberately restricted.

Can fasting cause muscle loss?

Weight loss can involve some loss of lean tissue. Adequate protein and resistance training are important for preserving muscle, particularly during calorie restriction and with increasing age.

Is intermittent fasting safe for older adults?

It may be suitable for some older adults, but the evidence base in older populations is less developed than in younger adults. Nutritional adequacy, protein intake, medications and individual health circumstances should all be considered.

Is fasting better than calorie restriction?

Not necessarily. Both can produce weight loss. Recent evidence suggests that intermittent fasting is generally effective, but usually offers only modest advantages over conventional continuous calorie restriction.

You may be interested to check out our post on detoxing your body.

Medical Disclaimer

This article is intended for general educational purposes and is not medical advice. Intermittent fasting is not appropriate for everyone. People with medical conditions, people taking medications affected by food intake or blood glucose, and anyone considering a significant dietary change should discuss it with an appropriate healthcare professional.